Impacts of Climate Change on Pacific Northwest Conifers

Ettinger et al. (2011) write that although the ranges of some species have moved upward or poleward in response to past periods of warming, "as expected," they note that "others remain static or have shifted in the opposite direction," citing Parmesan (2006) and Harsch et al. (2009).

In an attempt to shed more light on this intriguing situation, the three University of Washington (USA) researchers examined growth-climate relationships for six conifer species, consisting of three high-elevation species (Abies amabilis or Pacific silver fir, Callitropsis nootkatensis or Alaskan yellow-cedar, and Tsuga mertensiana or mountain hemlock) and three low-elevation species (Pseudotsuga menziesii or Douglas-fir, Thuja plicata or western red-cedar, and Tsuga heterophylla or western hemlock), both of which groups occupy contrasting altitudinal ranges on Mt. Rainier (Washington, USA), where they collected over 90 years of annual tree growth data from over 600 individual trees growing at nine different elevations, which results they compared with various components of long-term climate records. So what did they learn?

Ettinger et al. report finding that "contrary to common assumptions, climate does not determine all range limits." They say that it "strongly constrains performance at upper limits of conifer species reaching treeline, but more localized processes drive growth at upper range limits within closed-canopy forests." In harmony with Menge and Sutherland (1987), therefore, they conclude that "biotic interactions are likely to be strong in closed-canopy forests, and may constrain performance more than climate." And in harmony with Brown et al. (1996), they say that "the distribution of Pacific Northwestern conifers may therefore conform to the hypothesis that physiological tolerance limits species distributions where climate is harsh, but biotic interactions affect distributions where climate is not stressful."

Contrary to what many enamored with the "climate envelope" approach to predicting range responses to climate change have long contended, Ettinger et al. conclude that "the lack of climatic constraints on all range limits suggests that range shifts in a time of climate change will be difficult to accurately predict, particularly in closed-canopy forests where biotic interactions may be important range determinants."